The present invention relates to latch assemblies, and in particular latch assemblies for use with car doors and car boots.
Latch assemblies are known to releasably secure car doors in a closed position. Operation of an inside door handle or an outside door handle would release the latch, allowing the door to open. Subsequent closure of the door will automatically relatch the latch. Electric actuators are commonly employed in car latches in order to release them. Known latches incorporate a rotatable claw which engages with a striker mounted on an opposing surface (for example, a car door frame) in order to retain the door in a closed position. This rotating claw is often held in position by a pawl, which is also often a rotating component. Release of the claw is thereby achieved by rotating the pawl from an engaged position, whereby it engages and retains the claw, to a disengaged position, whereby the claw is free to rotate. Movement of the pawl is often undertaken by electric actuators. It is desirable to reduce the amount of force required to move the pawl from an engaged position to a disengaged position such that the size of the electric actuator can be reduced, thereby reducing weight and part cost.
Simple known latch assemblies include a pawl that is mounted to rotate about a single axis. Such pawls are rotatably mounted on a substantially cylindrical pawl pivot pin inserted into a circular pawl pin orifice in the pawl. The pawl pivot pin is fixed to a stationary latch chassis. The pawl pivot pin has to be of a certain radius in order to withstand loads that the latch may undergo during normal operation and also during high load impact events.
A problem with this type of known latch is that a radius of the pawl pivot pin, which as described must be of a certain magnitude to withstand loads, is directly related to the size of the contact area between the pawl and said pawl pivot pin. This is problematic as the amount of friction between these two components is influenced by the amount of dust and contaminants that may accrue between them. Therefore, as the contact surface area is increased, the levels of friction inherent within the latch in use is also increased, and a greater actuation force is required to overcome such friction. Therefore, larger and more expensive actuators are required which is undesirable.
GB2409706 shows an example of a low energy release latch 100 (as shown in FIG. 1) including a first pawl 140 pivotally attached to a toggle link 130, and also to a second pawl 160 configured to retain the toggle link 130. A high level of force acts on the first pawl 140 as a result of the vehicle door seal load, driving the claw 120 in a clockwise direction. The seal load acts to collapse the toggle link and pawl arrangement as shown in FIG. 8, which is prevented in FIG. 1 by the interaction of the first pawl 140 and the second pawl 160. Release of the low energy release latch 100 is therefore achieved by a clockwise rotation of the second pawl 160, which in turn releases the first pawl 140.
WO/2006/087578 discloses a device (see FIG. 1), in which the first pawl 16 is mounted on a crankshaft 50. Door seal loads act to rotate the rotating claw 14 in a clockwise direction, which rotation is prevented by the first pawl 16. The first pawl 16 is mounted on a crankshaft 18 and is configured such that force FP acts to generate a clockwise torque on the crankshaft 18, which is rotatationally constrained by a release plate 72 acting on a release lever 52 (see FIG. 1B). Release by actuation of the release plate 72 allows the crankshaft 50 to rotate and the pawl to move under force FP to enable the latch to open.
It can be clearly seen in WO/2006/087578 that the radius on which the first pawl 16 rotates about a crank pin 54 is necessarily large in order to encompass a cylindrical pin 56 (see FIG. 1C). The radius of the crank pin 54 therefore has to be equal to at least the distance between the crank pin axis Y and the crank shaft axis A plus the radius of the cylindrical pin 56 (i.e., the minimum required radius rmin).
Such a large radius of rotation means that a perimeter of a pivot hole 46 is significant. Typically, the radius of the pivot hole 46 is in the order of 9 millimeters or more. This is problematic as dust contamination can cause excessive friction between the first pawl 16 and the crankshaft 50, increasing the effort required to rotate them relative to each other. This is undesirable as larger actuators are required to rotate the two components relative to each other.
Any attempt to reduce the radius of the crankshaft 50 to distances below the minimum required radius rmin would result in significant weakening of the crankshaft and consequently likely failure of this component.
Referring to FIG. 1 of WO/2006/087578, a torque is applied to an eccentric 54 as the line of action of force FP is offset from an axis A. The size of the lever arm at which this torque is applied is determined by the start angle of the eccentric 54 (i.e., in the closed position). By way of explaining what is meant by “start angle”, at start angles of 0 and 180 degrees, the eccentric 54 is at top dead center (unstable equilibrium) and bottom dead center (stable equilibrium), respectively. As the angle tends towards 90 degrees, the lever arm increases to a maximum, and the maximum torque for a given force FP is applied to the eccentric.
As the start angle decreases, the lever arm producing the torque on the eccentric 54 decreases. As such, if the angle is too low (i.e., below a minimum backdrive angle), the torque produced by the lever arm and the force FP will be insufficient to overcome the friction in the system, rotate the eccentric 54, and open the latch. In known latch arrangements, the start angle must be above the minimum backdrive angle, typically in the order of 54 degrees.
This minimum backdrive angle is indicative of the friction inherent in the latch assembly and therefore of the torque required to open the latch assembly. If it is reduced, a lower torque is sufficient to open the latch. This is beneficial as less effort is therefore required to release and latch the latch.